The present invention relates to an electronic current amplification and collection structure for photomultiplier tubes and to a photomultiplier tube incorporating such a structure. In particular the current amplification and collection structure includes a micro-channel plate multiplier and a reverse-biased semiconductor diode.
Photomultiplier tubes are known for detection or imaging of electromagnetic signals including signals of particular spectral characteristics such as infra-red signals, visible light signals, ultra-violet, x-rays, and gamma rays. In a typical photomultiplier tube, photons of such signals are incident upon a biased conductive surface, a photocathode, which emits electrons via the photoelectric effect. These primary electrons are then accelerated toward a biased conductor, or dynode, which emits further electrons, i.e., secondary electrons. Amplification is achieved within a photomultiplier tube by arranging several dynodes to receive incident electrons and to emit secondary electrons, and by configuring the biasing electric fields among the dynodes to guide the emitted electrons along paths between successive dynodes. Ultimately, the cascading stream of electrons is collected to provide an electrical current proportional to the incident photon flux. The degree of amplification provided between the initial photon flux and the collected electron current is determined by factors including the electron emission characteristics of the dynodes, the number of dynode stages, and the voltage applied between successive dynodes for accelerating the electrons.
It is desirable for photomultiplier tubes to provide as high an amplification as possible for a given applied voltage. It is also desirable for photomultiplier tubes to be compact and mechanically reliable. For imaging purposes, it is also desirable for the two-dimensional cross section of the amplified electron stream to accurately represent the two-dimensional distribution of incident photons.
One device for amplifying an electron beam while maintaining the two-dimensional distribution of the beam is a microchannel plate. For example, U.S. Pat. No. 5,086,248 to Horton et al. describes methods for producing a variety of microchannel plate structures formed from semiconductor wafers. A typical microchannel plate includes a body of secondary electron emissive material having a number of pores extending through the body. Electrodes formed on respective sides of the body allow application of a bias voltage parallel to the direction of the pores. In operation, incident electrons collide with the walls of the pores, thus causing a cascade of secondary electrons which further collide with the pore walls to provide amplification of the incident photon flux.
In accordance with one aspect of the present invention, a device for amplifying and collecting electron current in a photomultiplier tube is provided. The device combines a microchannel plate (MCP) formed of a semiconductor material and a planar, reverse-biased semiconductor diode for collecting electrons emitted from the microchannel plate. The MCP and reverse-biased diode may be provided as a monolithic structure by forming the MCP in a semiconductor substrate such that the channels of the MCP extend into the substrate to a predetermined depth, and by forming the diode to be located beneath the bottom of the channels.
In accordance with another aspect of the present invention, amplification and collection of an electron flux is enhanced by a structure incorporating a microchannel plate and a planar diode. The microchannel plate and diode are preferably formed monolithically. The microchannel plate amplifies an incident electron flux by emission of secondary electrons. The diode is configured to provide solid-state amplification by mechanisms of electron bombardment induced current (EBIC) and/or by avalanche generation of excess carriers.